Conventionally, studies are being made of color recording techniques based on electrophotographic printing, ink-jet printing, thermal transfer printing, etc., for the purpose of applying such color recording techniques to facsimile, copying machines, printers, or the like.
Among these recording techniques, the thermal transfer recording technique is thought to be more advantageous than the others because the maintenance and operation of the apparatus are easy and the apparatus and its expendable supplies are inexpensive.
There are two systems in the thermal transfer recording: fusion-type transfer in which a transfer recording sheet consisting of a base film and formed thereon a thermally fusible ink layer is heated with a thermal head to fuse the ink and transfer the fused ink to an image-receiving surface, thereby recording an image on the receiving surface; and sublimation-type transfer in which a transfer recording sheet consisting of a base film and formed thereon a colorant layer containing a sublimable dyestuff is heated with a thermal head to sublimate and/or heat-diffuse the dyestuff and transfer the dyestuff from the transfer recording sheet to an image-receiving surface, thereby recording an image on the receiving surface. The sublimation-type transfer is particularly advantageous to full-color recording over the fusion-type in that gradation recording is easy since the transferred amount of the dyestuff can be controlled by changing the energy fed to the thermal head.
In the thermal transfer recording of the sublimation type, sublimable dyestuffs used in transfer recording sheets and in ink compositions for producing transfer recording sheets greatly affect the speed of transfer recording, the quality and storage stability of prints, etc. Therefore, the sublimable dyestuffs are highly important and need to satisfy the following requirements:
(1) the dyestuffs should readily sublimate and/or heat-diffuse under operation conditions for the thermal head;
(2) they should not undergo thermal decomposition under operation conditions for the thermal head;
(3) they should possess tints favorable for color reproduction;
(4) they should have high molecular absorption coefficients;
(5) they should be stable to heat, light, moisture, chemicals, etc.;
(6) they should be able to be easily synthesized;
(7) they should have good suitability for use in preparing inks; and
(8) they should have no health and safety problems.
However, a dyestuff which meets all the above requirements has not been found so far. Yellow dyestuffs, in particular, have various defects and satisfactory one has not yet been developed.
Conventionally known yellow dyestuffs having suitability to thermal transfer recording of the sublimation type are disclosed, for example, in JP-A-61-244595, JP-A-60-27594, and U.S. Pat. No. 4,808,568. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) These yellow dyestuffs are pyridone-type monoazo dyestuffs having the same basic structure as that of the dyestuff employed in the present invention. However, even the dyestuffs whose structural formulae are specified in those references have had still insufficient performances.